In the almost 50 years since its invention, many improvements have taken place in electrophotographic print engines. In particular, dry printing or xerographic printing machines used in copiers have become a stable and necessary element of modern business life in the industrialized world. More recently, full color electrophotographic print engines have been produced and are becoming more popular as their price drops as compared to monochromatic machines.
Still more recently, the use of electrophotographic print engines in laser printing devices is becoming a common staple of many office environments because of the very high quality images produced thereby and their versatility with respect to type fonts, graphic reproductions, and the like.
A machine embodying significant steps in size and cost reduction for a full color electrophotographic print engine, usable in a copying machine, laser printer, or other image producing apparatus requiring full color electrophotography, is disclosed in U.S. Pat. No. 4,652,115 to Palm et al. issued Mar. 24, 1987. U.S. Pat. No. 4,652,115 is assigned to the assignee of the present invention. The Palm '115 patent disclosed what its inventors believed to be the first practical full color electrophotographic print engine employing flexible belts rather than drums as a carrier for the engine's photoreceptor. Additionally, the Palm '115 patent discloses a practical belt oriented full color double transfer electrophotographic print engine.
The toner transfer mechanism in the machine disclosed in the Palm '115 patent uses conventional coronas which are devices which have been used for many years in electrophotographic print engines for establishing high electrostatic fields for toner transfer and toner adhesion. As is well known to those skilled in the art, coronas are useful for creation of high electrostatic fields but their performance, particularly with respect to the fields created by a corona with a given input voltage and current, are greatly affected by ambient conditions of air temperature and humidity.
Thus, it is known that use of other devices to create electrostatic fields for toner transfer helps overcome perturbations in corona performance as a result of environmental parameters.
One such mechanism is the direct application of an electric field between a surface carrying a developed toner image and a surface to which the images to be transferred by means of application of a constant voltage between the mechanical devices (usually drums) carrying the two respective surfaces. An electrophotographic print engine employing such a mechanism is shown, for example, in U.S. Pat. No. 3,729,311 to Langdon.
Traditionally, relatively high source voltages are used in the transfer of toner material in electrophotographic print engines, particularly color engines in which transfers for a multiple layers of toner material must be made. For example, Langdon teaches transfer from photoreceptor carrying drum to paper carrying drum using an applied field of between 3,000 and 4,000 volts. Although the applied field is a relatively high voltage, the actual voltage across the paper is much less because there is a large voltage drop in the air between the corona and the paper carrying drum.
As is known to those skilled in the art, the danger of increasing applied transfer voltage in order to increase transfer efficiency comes from the phenomenon of electrical breakdown between the photoreceptor and image receiving web, which term includes an ultimate print receptor such as paper or a sheet of transparent material, or an intermediate transfer belt as in the case of the preferred embodiment of the present invention. Electrical breakdown arises when the electric field intensity at various points along the photoreceptor/image receiving web interface reaches a sufficient strength to ionize the material lying there between, usually gases in the air. Once a path of ionized gas is established, the electrical resistance between two points at the extremes of potential of the electric field becomes very low and available current will rush through the path of the breakdown.
Naturally, electric field intensities get very high at the photoreceptor to image receiving web interface in an electrophotographic print engine in which an applied field of several thousand volts is placed across a very narrow gap between these two surfaces.
As is known to those skilled in the art, selection of surface resistivity or conductivity of a surface at such an interface in a high electric field affects the total current which flows through breakdown paths. Once an ionized breakdown path is established, the total current flowing through the path will be determined by the surface resistivity of the material lying at the relative negative potential of the applied field, since this is the source of electrons. If the material in question has a low surface resistivity, there are a large number of electrons available and a high current will flow through the discharge path, possibly causing significant damage as it does so.
It is known to those skilled in the art that the total current through any given breakdown path will determine the destructive impact of the flow through the breakdown. Examples from everyday life illustrate this phenomenon. On the one hand, consider the commonly encountered spark from the human hand to a grounded metal surface (often a door knob) which one experiences when walking across a rug or similar surface which will induce a triboelectrical charge in the body.
Thus, the conventional wisdom has taught the use of insulating materials for construction of photoreceptors in electrophotographic print engines. Known belt constructions include those with relatively low resistivities covered by a highly resistive layer of material having a high dielectric constant. In either instance, the principle behind use of such a device is to severely limit the current available to flow through any given breakdown path. Breakdown and electronic discharge through toner materials in electrophotographic print engine are believed to cause small spots or flecks in areas where toner is removed due to the phenomenon of multiple small explosions around the breakdown. If sufficient current is available, the electrical discharge which results from the breakdown can damage the photoreceptor belt. This can be either a macro phenomenon in which there is physical damage to the belt or a micro phenomenon in which portions of the photoreceptor material are destroyed. The cumulative effect of large numbers of breakdowns ultimately degrade photoreceptor performance to an unacceptable level.
Therefore, the prior art shows use of highly insulative materials for photoreceptors and backing media on ultimate image receptors to limit current which flows through discharge paths.
It is the surface resistivity characteristic, normally specified in ohms or ohms per square, which primarily impacts the above described available current in the event of a discharge. It is also a very high surface resistivity which tends to cause the accumulation of local charge maxima.
Although it turns out that the magnitudes of the preferred ranges for both surface resistivity and bulk resistivity on the transfer belt used in the preferred embodiment are substantially identical, this is a coincidence. It is known that one can engineer belt and drum materials in a fashion in which the numerical value of the bulk resistivity (given in ohm centimeters) substantially differs from that of the surface resistivity (specified in ohms).
There is a countervailing consideration which results from the use of materials of very high surface resistivity for these applications. The problem which results from the use of materials of high resistivity is that areas of very high charge will accumulate locally on the surface of such a material and will not dissipate, or spread out evenly over the surface of the photoreceptor, because of its very low conductivity. Increasing the conductivity of the material allows these local maxima of charge to dissipate and thus provides a much more uniform charge per unit area characteristic to be established.
Additional problems are presented in full color electrophotographic print engines in which multiple toners are used to build up a composite image which is ultimately fixed to the image receptor. One problem which is known arises from the effects of previously deposited layers of toner on the transfer of the second and third toners to be laid down in forming a composite image. It is well known that toners are normally plastic particles having specified triboelectric charge characteristics, which characteristics have the dimensions of charge per unit mass. The toner or developer particles are physically agitated and accumulate triboelectric charge thereon of a specific polarity. Also, it is well known that toner materials can be manufactured to specified average triboelectric charge characteristics, which are normally stated in microcoulombs per gram. Thus, after sufficient agitation, the average triboelectric charge present within a collection of toner material will be a certain number of microcoulombs per gram.
It is known in the art that judicious selection of average triboelectric charge characteristics for toner materials can contribute significantly to overall print engine performance, whether in a copier, laser printer, or other device employing such a print engine. The parameters impacted by triboelectric charge are primarily development density, i.e. the efficiency in properly developing latent electrostatic images with toner materials, and transfer from the original photoreceptor to the ultimate image receptor used in the machine. The latter may be accomplished by directly transferring the image onto the ultimate print receptor, normally a sheet of paper, or through the use of an intermediate transfer drum or belt, as is disclosed in the preferred embodiment of the present invention.
Additionally, triboelectric charge for toner materials impacts a phenomenon known as back transfer in multipass electrophotographic print engines. The phenomenon of back transfer refers to any tendency of toner materials (deposited on an image receiving web during a previous transfer of a developed separated image) to move back to the image carrying web from which such materials were originally transferred during transfer of a subsequent collection of different toner particles. In other words, if a first separated image is developed on a photoreceptor and transferred to a sheet of paper, the phenomenon of back transfer refers to the tendency of the first toner materials on the paper to transfer back to the photoreceptor as a second developed separated image is being transferred to the paper.
Naturally, there is a significant and intimate interplay between applied fields and toner triboelectric charge characteristics which ultimately determines the quality of a final printed image in an electrophotographic print engine.
The selection of appropriate triboelectric charge characteristics for toner materials becomes much more critical in full color copying machines than is the case for monochromatic machines for the following reasons. First, multiple toners (usually three) are used in full color copiers and printers and thus there is the opportunity for significant interaction between the various toner materials used to develop the individual separated images. Secondly, there is a significant factor with respect to the perceived quality of the ultimate output of the print engine because of their normal uses. A color print engine is required to develop a set of ultimate images of a much broader range than one normally encounters in the use of monochromatic machines, particularly those used primarily to reproduce text in an office environment. There is a significant dynamic range with respect to color saturation, and a significant range of the spectrum which such machines are required to reproduce. Also, the perceived quality of the output of a full color print engine is greatly diminished by spectral inaccuracy and inappropriate pigment separations, particularly at image boundaries or in dark areas of the output produced by significant densities of all three toners.
In the first category, problems with respect to separated image registration in forming the final composite image can create significant problems with respect to the perceived quality of the output. Registration problems normally manifest themselves on edges of the image where individual pigments are reproduced along the boundary where a composite image of essentially uniform color is what is represented by the source of image information driving the print engine.
Additionally, the well known phenomenon of halo severely deteriorates the perceived quality of the ultimate image in a color machine, whereas the effect is of marginal significance in a monochrome print engine. In a color print engine, halo manifests itself as inconsistent pigment mixing at a boundary in an image even if registration of the separated images to form the composite image is done with virtual perfection. The visual effect is similar to that caused by misregistration. However, halo will normally appear as a consistent border of inappropriate pigmentation surrounding an entire image segment, whereas registration problems normally manifest themselves as visible shifts in the pigments from the separated images in a particular direction.
It is known in the art that certain undesirable effects in color electrophotographic print engines may be overcome, or offset, by selection of triboelectric charge characteristics of the toner materials. In particular, U.S. Pat. No. 4,093,547 to Hauser et al. discloses a color electrophotographic print engine in which stepped triboelectric charge characteristics for the three color toners are used in order to minimize the phenomenon of back transfer. Hauser discloses a single transfer color electrophotographic print engine in the environment of a copying machine. The images are developed using yellow, cyan, and magenta toner materials, respectively.
Hauser teaches a wide and decreasing sequential range of triboelectric charge characteristics for the three toner materials. In particular, he teaches a preferred embodiment of 44 microcoulombs per gram for the first toner (yellow), 20 microcoulombs per gram for the second toner (cyan) and 6 microcoulombs per gram for the third (magenta) toner.
The principle of operation of this machine is as follows. Charged areas of the photoreceptor which develop each separated image naturally attract the toner materials in use, which is how the latent image is turned into a developed image in the first place. Once a first layer of toner materials has been transferred from the photoreceptor, a subsequent pass to the photoreceptor will contain charged areas where the second image is developed. While an applied field is used to transfer toner materials in the desired direction, there is a significant electric field contribution from the charged areas of the photoreceptor which tends to draw the previously transferred particles back to the photoreceptor. This is particularly true at boundaries between pigmented areas where, for example, there may be a toner material of the first color already transferred to the paper and an absence of toner of the second color on one side of the boundary. The charged area of the photoreceptor near the boundary will tend to draw the previously transferred particles back to the photoreceptor, which lowers the density of the previously transferred separated image of the first pigment and may corrupt the composite image.
Next consider the situation when the second layer of toner has been transferred and a developed image with the third toner material is approaching the transfer station. First, areas of the composite image which contain both the first and second pigments will be closer to the photoreceptor, and thus the photoreceptor's pull on the top layer (the second toner material) will be stronger than the like attraction to the first pigment toner during the previous transfer. Similarly, it is believed that as the third layer is transferred, boundaries between charged and uncharged portions of the photoreceptor which are substantially perpendicular to the direction of travel of the photoreceptor can also tend to pull portions of the last deposited toner material back to the photoreceptor.
It is the teaching of Hauser that successively decreasing the triboelectric charge characteristics of the toner materials reduces the attraction between the photoreceptor and the later deposited materials having the lower charge.
For reasons set forth hereinbelow, the inventors of the present invention believe they have discovered that Hauser's approach is exactly the opposite of the optimum use of stepped triboelectric charge characteristics for toner materials. Therefore, one aspect of the present invention involves the use of stepped triboelectric charge characteristics from materials in the opposite order of that taught in Hauser, i.e. the last toner material to be transferred is the one of the highest triboelectric charge.
Also related to the problem of back transfer is the problem of non-uniform forward transfer. Often it is quite difficult for the designer or operator of an electrophotographic print engine to ascertain which phenomenon is causing problems in the quality of the printed output. In other words, if the toner materials never leave the photoreceptor in the first place, it normally makes little difference in the ultimate print quality whether they simply failed to leave when they were supposed to be transferred, or were transferred initially and back transferred to the photoreceptor on a subsequent pass. Thus, minimizing the phenomenon of back transfer as well as attempting to assure uniformity of forward transfer are important design goals for color electrophotographic print engines.
In this respect, the following should be noted. One way of approaching the problems is to postulate that the best transfer which takes place in a color electrophotographic print engine is that of the first toner material transferred from the photoreceptor to an image receiving web, either the final image receptor or an intermediate transfer web. This is because it is relatively straightforward to provide a uniform surface charge characteristic on the image receiving web when the first developed separated image is to be transferred thereto. Subsequent transfers of the second and third images encounter perturbations in the surface electric field characteristics caused by the already present toner materials from previously transferred images.
One good approach to overcoming the perturbations due to previously transferred toner materials is disclosed in U.S. Pat. No. 3,992,557 to Kubo et al. The Kubo apparatus uses a plurality of additional coronas to precharge the image receiving web so as to overcome perturbations in the surface charge characteristics of the web as a developed image approaches the transfer station. It should be noted that, in this context, the surface charge characteristics of the web include the contributions from previously transferred toner materials.
A primary drawback of the Kubo approach is expense, i.e. the use of several additional coronas, and the consequent environmental instability which comes from any device relying strongly on coronas to establish desired electrostatic fields.
U.S. Pat. No. 3,729,311 to Langdon (cited above) shows another useful method of assisting in uniformity of forward transfer characteristics in a full color electrophotographic print engine. Langdon discloses the use of a conductive backing for a drum carrying a sheet of paper (or other image receptor) and postulates that degradation in forward transfer results from an increased composite resistivity of the toner receiving surface (the paper and previously transferred toner layers) during transfer of second and third toner images. He further states that increasing the fixed bias between photoreceptor and the backing for the print receptor was an unsuccessful experiment, indicating that it was believed that breakdown phenomena prevented transfer of the images in question.
In order to overcome this, Langdon teaches the use of a stepped applied field as subsequent toner layers are transferred to build up the composite image. In particular, Langdon teaches a preferred range of 3,000, 3,500 and 4,000 volts as the applied field for three successive transfers of three toner materials. The preferred embodiment of the present invention also makes use of the principle of stepped applied fields during toner transfer.
As noted above, the phenomenon of halo is an unintentional and inappropriate color separation at image boundaries which is encountered in full color electrophotographic print engines. It is the belief of the inventors of the present invention that the phenomenon of halo has become so problematic in prior art color electrophotographic print engines because of an inappropriate application of certain accepted design principles applicable to monochromatic print engines to color print engines. In particular, it is the present inventors' belief that persistent use of toner materials of high bulk resistivity and high triboelectric charge exacerbates the problem of halo in a color print engine.
It is known to those skilled in the art of electrophotography that the use of high resistivity toner materials (usually having resistivities greater than 10.sup.10 ohm centimeters) enhances edge definition in monochromatic print engines but tends to deteriorate to some degree the fill characteristics of large densely decorated areas. This phenomenon is well known to users of monochromatic print engines wherein large black areas will often run to various shades of gray when the saturation of the original image was uniform. However, edge enhancement is achieved because highly resistive developer materials tend to accumulate small excesses of the materials at the boundaries. This tends to cause dense deposits of toner near the boundaries between black and white portions of the image and thus increases the perceived sharpness of the image, but at the expense of large area fill. Since a large percentage of monochrome electrophotographic print engines are used primarily to copy text, this has conventionally been viewed as a very acceptable trade off.
The present inventors have developed what they believe to be an accurate model for why the use of conventional resistive developer materials causes significant halo in color print engines and, applying the teachings of this model, have achieved printed output from a color electrophotographic print engine embodying the present invention with much less halo than is encountered in the prior art without sacrificing fill density. In particular, it is well known to those skilled in the art that one encounters rotations of the electric field gradient in the field between a toner brush at a development station and a charged photoreceptor carrying a latent image. The rotation of the field gradient occurs around boundaries between highly charged and discharged portions of the photoreceptor corresponding to dark areas of the image and light areas of the image, respectively. The rotation of the field gradient causes an excess of toner to accumulate at the boundary and thus explains the above described phenomenon of increased density at boundary areas in monochromatic electrophotographic machines.
The high resistivity of such toner materials also explains the absence of good fill in large dark areas. The highly resistive developer materials, like any material of very high bulk resistivity, cannot distribute local maxima of charges due to their lack of conductivity. Thus, there tends to be small localized areas of varying triboelectric charge density on the developer brush. Therefore, toner is transferred to the photoreceptor with a non-uniform density during development of large areas. It should be noted that this phenomenon rarely causes any visible degradation in a text image since slight variations between the densities of adjacent letters are imperceivable, but varying densities of a continuous dark area are less attractive and quite noticeable.
The conventional wisdom of prior art monochromatic electrophotography is that decreasing the resistivity of the toner materials improves the fill problem but at the expense of sharpness on the image boundary.
The present inventors believe that significant halo encountered in conventional full color electrophotographic print engines results in large part from the use of highly resistive developer materials. In particular, it is believed that the resistive developer materials cannot dissipate the local maxima of charge at developed image boundaries which results from the gradient rotation at the boundary, which in turn results from accumulations of excess toner materials at a boundary. Since the toner materials are so resistive, the excess of toner materials near the boundary tends to create an excess of charge on the first and second layers transferred. Transfers of subsequent layers encounter a higher electric field contribution from these local maxima near the boundary field which, the inventors believe, causes dissipation of the toner near the boundary during subsequent transfer steps due to field gradient rotation.
Lastly, it is known to those skilled in the art that it is more difficult to produce good black coloration by using the composite pigment resulting from three individual pigments than by using black toner materials of the type used in monochromatic print engines.
To summarize much of what is discussed above, the art of full color electrophotography is significantly more complex than that of conventional monochromatic electrophotography. The complexity arises primarily from the problems encountered in transferring multiple toner images, and from the greater sensitivity of the human observer to image distortions in the final printed output for a color image than to physically similar distortions in a monochromatic image. From a practical point of salability and utility of color electrophotographic print engines, users have higher expectations from color electrophotography than monochrome electrophotography. This is due to both human sensitivity to color images and the higher price of color machines.
Most problems (other than registration) arising in image quality in full color electrophotography are caused by classes of phenomenon related to the effects of earlier transferred toner images on the transfer of subsequently transferred images. These include non-uniformity of forward transfer due to field non-uniformities and back transfer phenomena. The art of designing color electrophotographic print engines is, in large part, the problem of attempting to understand the physical mechanisms involved and to select appropriate trade offs among parameters which impact the ultimate image quality.